Undefined function t for autonomous ODE

Question

David 2014-7-11

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/141446-undefined-function-t-for-autonomous-ode

评论： Sara 2015-2-17

I am new to matlab, and I am attempting to learn about modeling with ODE's with this helpful tutorial:http://matlabgeeks.com/tips-tutorials/modeling-with-odes-in-matlab-part-3/

But I am having problems running the code here. I don't know if it was created in a different version or what. I am using R2013b.

Below is the code all together with comments. When I run this, I am first prompted for input arguments. It is suggested that I run it as lokta_volterra(t,x). However, when I do this, I get Undefined function or variable 't'.

Now of course t is not "used" as this is, as the comments state, this is an autonomous equation. But I'm not sure how I'm supposed to run this to display the graph indicated on the help site.

% lotka_volterra.m
%
% Imlements the Lotka-Volterra function
%   dx/dt = alpha x - beta xy
%   dy/dt = delta xy - gamma y
%
% Inputs:
%   t - Time variable: not used here because our equation
%       is independent of time, or 'autonomous'.
%   x - Independent variable: this contains both populations (x and y)
% Output:
%   dx - First derivative: the rate of change of the populations
function dx = lotka_volterra(t, x)
  dx = [0; 0];
  alpha = 1; 
  beta = .05; 
  delta = .02;
  gamma = .5;
    dx(1) = alpha * x(1) - beta * x(1) * x(2);
    dx(2) = delta * x(1) * x(2) - gamma * x(2);
    % Set our preferences for ode45
  % The default relative tolerance is 1e-3.
  % To set our output to non-negative, we provide an array listing
  %   each population that we want to constrain.  Since this example
  %   has two populations, we pass the array [1 2]
  options = odeset('RelTol', 1e-4, 'NonNegative', [1 2]);
  % Use ode45 to solve our ODE
  % Place the time points in a vector 't'
  % Place the solution in a vector 'x'
  [t,x] = ode45('lotka_volterra', [0 20], [10 10], options);
  plot(t,x);
  legend('prey', 'predators');

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Answer 1

Sara 2014-7-11

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/141446-undefined-function-t-for-autonomous-ode#answer_144658

在 MATLAB Online 中打开

You need to put this part in a separate function and run the new function you create

options = odeset('RelTol', 1e-4, 'NonNegative', [1 2]);
  % Use ode45 to solve our ODE
  % Place the time points in a vector 't'
  % Place the solution in a vector 'x'
  [t,x] = ode45('lotka_volterra', [0 20], [10 10], options);
  plot(t,x);
  legend('prey', 'predators');